This invention relates to muscle stimulation systems for paralyzed persons. More particularaly, the invention relates to apparatus and method for stimulating controlled contraction of a muscle buried deeply below the skin of a paralyzed person without stimulating contraction of an overlying superficial muscle.
Typical prior art devices for stimulating paralyzed muscles are described in Petrofsky et al U.S. application Ser. No. 444,647, filed Nov. 26, 1982 and in other references cited therein. These prior art systems have a set of three electrodes for each muscle group to be stimulated. The electrodes are placed on the surface of the skin above the muscle group to be stimulated and are excited by pairs of pulsed stimulating signals. One of the three electrodes is connected to a high voltage ground, and voltage pulses are applied between that electrode and the other two electrodes in alternating fashion. The series of pulses applied between the ground electrode and either of the other electrodes occur at a frequency of about 60 Hz, and these pulses are alternated with 60 Hz pulses applied between the ground electrode and the other active electrode. The pulse width is disclosed as being about 500 microseconds, and the pusle amplitude varies in accordance with the desired stimulation level up to a maximum of about 255 volts. Such stimulation produces recruitment of all motor units and results in maximum effort by the muscle. Feedback signals are provided in order to control the amplitude of the applied stimulation signals in an automatic manner.
The above-described stimulation technique has been utilized only for stimulation of leg muscles. Leg muscles have been so stimulated for operation of exercise equipment, for pedaling vehicles and for walking. The muscles which have been stimulated have been the quadriceps, iliac, gastrocnemius and hamstring groups. These muscles are relatively large and are easily accessible for surface stimulation.
It has been found that when prior art stimulation techniques are applied to the stimulation of a hand, serious problems arise. Hand flexing is controlled by the flexor digitorum profundus manus muscle group which is relatively small and which is deeply located below superficial muscles controlling other motions. The same is true of the adductor pollicis muscle group which controls thumb flexing and the extensor carpi ulnaris muscle group which controls wrist extension for hand opening. The prior art stimulation system has been found unsatisfactory for stimulating those muscles, because the desired muscle stimulation is accompanied by stimulation of the overlying superficial muscles. Accordingly, there has been a need to provide a stimulation system which is able to stimulate deeply buried muscles without stimulation of the overlying superficial muscles.